1. Field of the Invention
The present invention relates generally to cellular wireless communications and more particularly to a code division multiple access (CDMA) cellular wireless communication system having an underlay system and a direct spread (DS) overlay system that occupy common spectrum and with the overlay system operating at least partially orthogonally to the underlay system.
2. Description of the Related Art
Cellular wireless communication systems are generally known to include a plurality of base stations dispersed across a geographic service area. Each of the base stations includes at least one antenna and a base station transceiver system (BTS) and provides wireless service within a respective cell. The BTSs couple to base station controllers (BSCs) with each BSC serving a plurality of BTSs. The BSCs also couple to a mobile switching center (MSC) which interfaces to the Public Switched Telephone Network (PSTN) and other MSCs. Together, the BTSs, BSCs and the MSC form a wireless network which provides wireless coverage to mobile stations (MSs) operating within a respective service area.
Wireless communication systems operate according to various protocol standards. One particular protocol standard in place worldwide is the CDMA protocol standard. CDMA is a direct sequence spread spectrum system in which multiple spread spectrum signals are transmitted and received simultaneously over a common frequency band. In the CDMA system, each mobile station (MS) may be assigned a distinct Walsh code which identifies the signals transmitted to and received from the MS.
In an example of operation thereunder, forward link signals from a BTS to a first MS are spread with a first Walsh code and then transmitted where the process of transmission includes pseudo noise (PN) scrambling (spreading). Likewise, forward link signals transmitted from the BTS to the second MS are spread with the second Walsh code and then transmitted, perhaps concurrently with transmissions from the BTS to the first MS. The first MS's receiver receives at its antenna all of the energy transmitted by the BTS. However, because Walsh code channels are orthogonal, after despreading the received signal with the first Walsh code, the despreader outputs all the energy intended for the first MS but, due to orthogonality loss, none of or only a small fraction of the energy intended for the second, third, etc., MS. Likewise, the second MS despreads the received forward link signal with the second Walsh code to receive its intended forward link energy. Each of the MSs then operates upon the despread signal energy to extract data intended for the respective MS. The number of users accommodated on the forward link is limited by intra-cell interference due to orthogonality loss, inter-cell interference and other interference such as that due to thermal noise.
In some specialized applications (e.g. fixed access) the reverse link channels may be configured such that Walsh codes separate reverse link users. However, in the typical case, the reverse link is strictly interference limited. In such special cases wherein signals are configured using Walsh codes, operation on the reverse link from the MSs to the BTS is similar to operation on the forward link. A plurality of MSs transmit to the BTS simultaneously on the reverse link with each reverse link transmission spread by a respective assigned Walsh code. A receiver of the BTS receives the composite reverse link signal and despreads the reverse link transmissions with expected Walsh codes to extract signals received from the first, second, third, etc., MS. The BTS then operates upon each despread signal to extract data sent by the MSs.
Because signals intended for other users of the CDMA system may appear as noise to other users due to orthogonality loss, because they are from another cell or because they are not orthogonal, CDMA capacity is interference limited. The number of users that can use the same spectrum and still have acceptable performance is determined by the total interference power that all of the users, taken as a whole, generate. Thus, the number of users that may be supported by each BTS on any frequency spectrum is limited. To increase the capacity of CDMA systems, additional base stations may be added to increase the number of cells within the service area. However, because load is often concentrated in a small geographic area, even with the addition of cells, particular cells may remain overloaded while neighboring cells are lightly loaded.
Thus, solutions have been proposed to overcome overcrowding in CDMA systems. One solution includes assigning multiple carriers within a single service area, with each of the multiple carriers servicing respective overlaying cells. In multiple carrier operation, each carrier is assigned a segment of the available spectrum which does not overlap other assigned segments of the available spectrum. Some mobile stations are serviced on one of the carrier frequencies while other of the mobile stations are serviced on other of the carrier frequencies.
Another method of improving capacity is to deploy carriers with wider bandwidths. This improves system capacity due to better spectral characteristics of the wideband users. However, interoperability of basic carrier systems and the wideband carrier systems would be problematic since the need to deploy both systems in the same frequency spectrum would arise.
A particular solution that addresses the interoperability issue is direct spread (DS) overlay wherein an overlay system operates atop an underlay system so that the overlay and underlay systems at least partially share frequency spectrum. In CDMA systems which employ orthogonal transmissions, an important element is an attempt to maintain orthogonality of transmissions. However, heretofore, preserving orthogonality has not been accomplished in an underlay/overlay system. Thus, minimal benefit was provided in having a DS overlay system.
Thus, there exists a need in the art for an underlay/overlay CDMA system in which overlay is employed but in which an overlay system has minimal adverse affect on the underlay system and vice versa.